It is often desirable to provide the operator of a vehicle with a map showing the topographical features of the terrain immediately surrounding the vehicle. This is particularly true for the pilot of an aircraft where the terrain may be obscured by cloud cover or darkness. Some of the advanced radar systems are capable of generating an image of the terrain immediately surrounding the aircraft. The generated map contains only limited information, and the radar image must be correlated with a navigational map in order to provide the pilot with the necessary information, such as labels identifying pertinent topographical features and recognizable landmarks. Alternatively, map displays have been developed in which the map is stored on one or more positive microphotographic transparencies such as taught in German Pat. No. 26 25 597, Richardson, et al. in U.S. Pat. No. 3,652,836, and Aron, et al. in U.S. Pat. No. 4,427,994. These microphotographic transparencies are projected in the operator's field of view and are capable of being moved in two orthogonal directions to keep the vehicle's coordinates in the center of the displayed portion of the map. In some of these systems, the transparency or even the whole projector system may be rotated so that the top of the displayed map coincides with the heading of the vehicle.
Evans, et al. in U.S. Pat. No. 4,179,693 disclose an autonomous check point navigational system for an airborne vehicle in which flight path is periodically checked and corrected by comparing the composition of features of an associated electronically stored reference map. The system electronically stores a plurality of reference maps, one for each check point and the addresses of the detected terrain features are modified to correct for an angular misalignment between the sensed map and the reference map.
An electronic moving map display such as disclosed by Seitz, et al. in U.S. Pat. No. 4,484,192 overcomes the disadvantages of the projected map and check point navigational systems and provides for increased versatility. Such electronic maps have been dependent on the storage capacity of electronic memories. Where electronic map data is compressed, required memory capacity has been reduced at the expense of difficulty in achieving real-time regeneration of a detailed map structure from the compressed data. The data compression techniques which traditionally have offered the greatest compression ratios unfortunately have generally required difficult decoding algorithms. Specifically, a polynomial compression method wherein a curve is stored, not as a multiplicity of points but instead as coefficients to a standard equation, offers a high compression ratio. This applies especially for lines and curves, which convey much of the useful map information. Unfortunately such a polynomial requires the evaluation of exponential terms in order to regenerate the data, requiring either extended processing time or extensive specialized hardware.
In addition, an electronic map would in theory allow the viewer to manipulate the orientation of the map in three dimensions, with six degrees of freedom and full perspective and relief. The transformation equations (such as those involving rotations) have been especially difficult to solve at video ("pixel") rates since the equations involve trigonometric functions and various multiplications. Then, in order to compress the transformed video data into two dimensions (for display on a conventional cathode ray tube), it has been necessary to use additional projection equations which involve division operations which must be performed on the results of the transform equations. Not only does this method require the use of multiplication, division, and trigonometric operations, but certain operations must be performed serially; that is, the division operation must be performed on the results of the previous trigonometric and multiplicative steps.
It would therefore be highly desirable to replace this known mathematical sequence with another (perhaps approximate) method which could achieve similar results, to the limit of cathode ray tube resolution, at conventional video pixel rates.